(1) Field of the Invention
The present invention generally relates to an electrostatic actuator continuously moving an object by using an electrostatic field, and more particularly to an electrostatic actuator having ferroelectrics in which residual dielectric polarization is formed. This type of electrostatic actuator is used, for example, as a flat electrostatic motor for straightly moving a plate-shaped object.
(2) Description of the Related Art
For example, International Patent Publication No. WO91/16757 discloses the following electrostatic actuator. This electrostatic actuator is formed of a stator having an insulator layer and a movable body having a resistive layer. On the insulator layer of the stator, a plurality of strip-like electrodes are formed so as to be arranged at predetermined intervals in parallel to each other. The movable body is on the stator under a condition in which the insulator layer and the resistive layer is in contact with each other. In this electrostatic actuator, voltages having a predetermined pattern (+V, -V and 0 volts) are applied to the strip-like electrodes. Due to electric fields formed by applying the voltages, bound electric charges (positive charges and negative charges) arranged in the predetermined pattern are induced in the resistive layer of the movable body. When the voltages applied to the strip-like electrodes are changed in accordance with a predetermined pattern, the attraction and repulsion between the bound electric charges induced in the resistive layer of the movable body and the electric fields formed by the strip-like electrodes are repeated. In this process, the movable body moves on the stator.
In the conventional electrostatic actuator described above, the bound electric charges induced in the resistive layer of the movable body is not maintained permanently. Thus, while the movable body is moving on the stator, the voltages must be applied to the strip-like electrodes at predetermined intervals to induce the bound electric charges. A time necessary for inducing the bound electric charges depends on a value obtained by multiplying a volume resistivity and a dielectric constant of the resistive layer together. If this value is large, a time for which the bound electric charges are maintained in the resistive layer is long. In this case, although a number of times of operations in which the voltages are applied to the strip-like electrodes to induce the bound electric charges in the resistive layer is decreased, the time necessary for inducing the bound electric charges in the resistive layer is increased in each operation. 0n the other hand, if the above value is small, the time necessary for inducing the bound electric charges in the resistive layer is decreased in each operation. However, since the time for which the bound electric charges are maintained in the resistive layer is decreased, the number of times of operations in which the voltages are applied to the strip-like electrodes must be increased. Thus, in both the cases, it is difficult to continuously and smoothly move the movable body at a high speed.
In addition, in a case where the bound electric charges are induced in accordance with the above principle, in order to increase a charge density of the bound electric charges so that sufficient drivability is obtained, relatively high voltages (+V and -V) must be applied to the electrodes. Thus, the insulation performance of the insulating layer of the stator must be improved. Furthermore, a switching operation of the high voltages must be performed at a high speed to move the movable body. Thus, electric characteristics of a voltage switching circuit must be improved.